Stack Overflow requires that questions be largely objective, but is based around a voting system that would be largely unnecessary if all questions actually were. The requirement for objectivity can be readily tested by asking a subjective question and watching how quickly it’s removed, but it’s also made explicit in their blog posts:
Stack Exchange is about questions with objective, factual answers.
…open-ended questions diminish the usefulness of our site…
and the reasons a question might be closed:
We expect answers to be supported by facts, references, or specific expertise, but this question will likely solicit debate, arguments, polling, or extended discussion.
The message is quite clear: objective good, subjective bad.